Have you ever wondered why releasing air from a balloon propels the balloon forward? Rockets work like a balloon filled with air. If you fill a balloon with air and shut the hole, the pressure inside the balloon is slightly higher than the atmosphere that surrounds it. The internal pressure inside the balloon is equal in all directions.

The moment you let go, air will escape through the hole and the internal pressure is no longer equal. The internal pressure in front of the balloon will be greater than the internal pressure near the hole. The balloon moves in the opposite direction of the escaping air. The net force that propels the balloon is called thrust. Rocket motors generate thrust by expelling a propellant out of a nozzle.

Figure 1: In equilibrium the balloon’s hole is closed. In the second stage, it is open. Thrust is a force that is a result of air escaping from the balloon. The image was created by the National Air and Space Museum and Smithsonian Institution.

For a rocket to launch, the thrust must be greater than its weight. Thrust counters the Earth’s gravity and lifts the rocket off the Earth’s surface. The greater the thrust, the faster the rocket will accelerate. Part of what determines the rocket’s acceleration is the momentum (momentum= mass x velocity) of the escaping particles.

When you release a balloon it doesn’t travel in a straight line. Unlike balloons, rockets have fins. Large fins positioned at the bottom of the rocket provide stability, constraining the movement in a straight line. When an object with fins (or wings) moves through the air, the fin deflects the air. Lift is a force generated by deflected air that makes airplanes fly.

Squid use thrust to move in water. They create thrust when they fill their mantle with water (Figure 2) then eject it. O’Dor et al. refer to the movement of squid from the water into the air as “rocket propulsion”. In this article O’Dory use Maciá’s photographs to explain how and why squid fly.

Squid anatomy. The squid fills it’s mantle with water and ejects it. Squid locomotion underwater is driven by thrust.

Methods

The photos of the squid were taken off the coast of Brazil by amateur photographer Bob Hulse. The photos were taken in burst mode (each picture was .15 seconds apart) with a Canon 1D camera and 100-400 mm lens (Figure 3). The main challenges in the study were 1) determining the size and age of the squid in the photographs and 2) determining how much energy the squid requires for rocket propulsion.

Figure 3: Some of the pictures used to understand squid flight. Pay attention to #1 in A, then look at #1 in C. Note the stream of water behind it. The squid uses the water to propel itself into the air, much like a rocket.

The squid were identified as S. pteropus by their location and shape. From the photos alone, O’Dor et al. could not estimate the squids’ size because there was no scale for reference in the picture. For example, you can take a picture of a bug next to a quarter to have an estimate of its size using the quarter as a scale. To understand how squid rocket propulsion “works”, O’Dor et al. reviewed literature about D. gigas and S. oualaniesis flight and examined specimens of Loligo and Illex species (very similar to S. pteropus) at the Smithsonian Museum of Natural History in order to confirm the presence of fin flaps.

Results

Squid flight has the following stages: 1) while underwater, the squid opens its mantle to draw in water, 2) it ejects the water using the water as a propellant, like a rocket, generating enough thrust to get out of the water and into the air, 3) it uses its fins and tentacles to deflect air by positioning them at a favorable angle that creates lift, and 4) it closes its tentacles upon re-entry into the water.

Figure 4: The parts and dimensions of the squid used to understand how squid fly.

Water is stored in the space noted as rm, shown in Figure 4. The squid is able to control the size of the opening, shown as ff. By making the opening smaller and exerting the water out with the same amount of force the squid is able to control its thrust by increasing or decreasing its acceleration. Based on the dimensions of the squid and some theoretical calculations, the scientists know that it is possible for the squid to generate enough thrust to propel it out of the water and through the air. The fin flaps, shown as lf on the right side, can act as wings. The squid can control the angle of the wing and as a result, produce an angle favorable for lift. From the photos alone, O’Dor and his colleagues were unable to see the flaps. However, they were able to estimate the distance the squid travels by flight. If the squid is assumed to be an adult, the squid can fly a distance up to a meter. The top speed appears to be 3.5 meters per second.

The scientists also noticed that the posture and color of the squid about to launch in the photo is different than those that are flying or are swimming in the water. O’Dor et al. suggests that squid not only decide to fly, but also prefer to do it in groups.

The most interesting conclusion made by O’ Dor et al. is that the energy required for a squid to fly is 1/5th of the energy needed to propel itself in water! Less energy is required to move through air because there is less drag moving though air than water. Air jet propulsion is favorable for squid because they are light in weight. There have been cases where tracked squid are traveling much farther distances than expected (based on estimates of swimming speeds). O’ Dor suggests that squid fly to travel farther distances as a way to save time and energy.

Significance

Squid have gained more attention as their role in pelagic ecosystems is better understood. I couldn’t find any video footage of squid flying, so maybe you can be the first the person in the WORLD to document squid in flight. What do you think?

Cat Turner is a Masters Candidate at the University of Rhode Island. Her research topic is on pH and dissolved inorganic carbon (DIC) fluctuations of Narragansett Bay, R.I. In her spare time she draws cartoons, reads horror stories, and collects wine corks. She likes to sail in fair weather.